The present invention relates to an electro permanent magnetic work holding apparatus for holding workpieces magnetically and/or mechanically. This invention more particularly relates to an electro permanent magnetic (hereinafter abbreviated as ‘EPM’ in this specification for the sake of brevity and convenience) work holding apparatus having a monolithic ferrous working face with slots of desired shape and configuration separating the magnetic poles, thereby rendering the apparatus suitable for holding the workpieces both magnetically and/or mechanically in the course of working/machining/anchoring. Usually such slots are made around the perimeter of the poles, thereby minimizing reduction in active magnetic area of the working face. The subject invention also pertains to a work holding apparatus with at least one non-reversible permanent magnet, and at least another reversible permanent magnet suitably placed, which may be commissioned for work holding by switching electrically, whereby a workpiece material is held in place without the use of additional pole separators.
In metal working machines such as CNC machining centers, milling machines, electrical discharge machines (EDM), grinding machines, etc., a magnetic work holding apparatus is often used as a worktable, whereby a workpiece to be machined can be held securely on the worktable by magnetic force, and then machining operation may be performed on the workpiece.
The principal property of a magnet is its capability to attract ferromagnetic materials resulting from flow of magnetic energy called “flux” between magnetic north and south poles. When a ferromagnetic workpiece is placed across the poles of a magnet, the “flux” passes through and the workpiece gets attracted. The intensity of attraction or pull becomes stronger with the decrease in the distance of separation between the workpiece and the magnet. Moreover, strength of attraction of a magnet is the function of the quantum of induction of magnetic flux into the workpiece. It has also been observed that smooth surfaces are better attracted and held in position in comparison to uneven or rough surface.
In the prior art, Electro permanent magnetic (EPM) work holding apparatuses of flux reversal type are known in which a magnetic circuit is activated or deactivated by reversing the poles of the permanent magnets of the device. This is a combination of permanent magnetic and electro magnetic devices: it presents the advantages of the latter without the disadvantages of the former. These devices uses intrinsic energy of the permanent magnetic device but instead of being switched “ON” or “OFF” mechanically, it requires electrical pulse similar to electro magnetic devices but only momentarily delivered by an electrical winding. Once switched “ON”, these devices provide magnetic force for infinite duration of time independent of any external energy source. For instance, U.S. Pat. No. 4,507,635 granted to Michele Cardone of Milan, Italy, pertains to a magnetic anchoring apparatus, comprising in combination: an external ferromagnetic crown provided with a base plate and lateral walls; at least one group of four pole pieces defining pairs of corresponding poles of an anchoring surface, said pole pieces presenting their longitudinal axes at right angles to the base place and in correspondence with the apexes of a square. Moreover, the apparatus comprises a plurality of permanent magnets for feeding the aforesaid poles, interposed between the pole pieces, and between the latter and said ferromagnetic crown.
Types of magnets presently available have magnetic poles (north and south) which are separated by non-magnetic insulators between the individual poles. The magnetic insulating material generally used may be selected from the group of epoxy, aluminum, brass, stainless steel, etc.
As the coefficient of thermal expansion of each material is different, the working surface of the existing magnetic work holding device is not stable when there is a rise in temperature during machining operation. This difference causes unstable surface and often creates small opening(s) for coolant (sometimes flooded coolant fluid is used during machining) to enter inside the work holding device and short circuit the winding/joints or hamper with the insulation resistance of the winding.
Normally in this conventional work holding device, all poles are individually machined and assembled, which gives rise to a possibility of the presence of weak points on the top working face. Moreover as multiple numbers of pieces are to be handled, it makes the manufacturing process difficult and time consuming.
It is impractical to repair these chucks as any process of repair cannot begin without destroying the chuck.
Moreover, another drawback of the conventional magnetic work holding apparatus is that it cannot be used for holding diamagnetic or paramagnetic workpieces, as a result of which they normally had to be removed from the working bed of the machine or a holding means such as a clamp or vice had to be mounted on top of the apparatus to hold diamagnetic or paramagnetic jobs or workpieces. Any other provision for clamping the workpiece also significantly reduces the active magnetic area of the working face.
As pointed out earlier, structural stability of prior art systems was not satisfactory, particularly while carrying on heavy machining applications. The filler material, usually epoxy compound, also was a source of problems, as it led to unequal heating of the work holding surface posing accuracy problems. Leakage of the coolant into the magnets affected the performance of the apparatus.
The present invention aims at circumventing and finding an effective solution to the difficulties mentioned above.